system-traffic: use `dpctl/*conntrack` instead of `conntrack` tool.

system-traffic: use `dpctl/*conntrack` instead of `conntrack` tool.

Often in the tests we inspect the conntrack tables with the 'conntrack'
command line utility.  Since this may not always be available, and since
these tests are supposed to run with the upcoming userspace connection
tracker, it is better to use the newly implemented dpctl command.

Due to the tcp state mapping done in tcp_state_coalesce(), SYN_RECV is
replaced by ESTABLISHED in four places in the testsuite.  The rest of
the changes are just done to match the formatting style.

Also, check the conntrack entries for the IPv6 HTTP test.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Joe Stringer <joe@ovn.org>